In the oil industry, various types of subsurface pumps are used for extracting crude oil from the reservoir to the surface. Among conventional artificial lift systems, the most prevalent type are mechanically driven subsurface pumps activated by means of a sucker rod string from the surface via beam pumping or other pumping units. Such pumps are capable of handling very high reservoir temperatures resulting from advanced recovery techniques (e.g., the injection of steam or in-situ combustion to lower the viscosity of the heavy and extra heavy crude oil). Due to the limited diameter of mechanical subsurface pumps and the number of strokes per unit of time at which they can operate, it is essential to achieve maximum volumetric efficiency at each pump stroke.
In mechanically actuated positive displacement subsurface pumps, the valve attached to the component that induces reciprocating motion is known as the traveling valve; while, the valve attached to the stationary component is known as the standing valve. The traveling and standing valves are basically retention valves arranged so that both allow fluid flow in the same direction. Consequently, the relative motion between these two valves produces the pumping action.
Mechanical pumps can be configured such that valve elements act as a plug and a seat, where fluid flows in one direction when the plug becomes separated from the seat by the pressure differential at both sides of the valve. The plug and seat may have any shape; provided that there must be a hermetic seal between them, in order to prevent reverse flow, when the valve is closed. Currently, the most commonly used configuration in the oil industry for the plug is a ball or sphere referred to as a “ball and seat” valve.
In order to allow interchangeability between manufacturers, the American Petroleum Institute (API) established the Standard API 11AX, which standardizes threads and tolerances of valve elements, but does not take into account the design nor the flow areas through the various components of subsurface pumps.
When pumping crudes with high gas to oil ratio, conventional subsurface pumps with ball and seat valves are somewhat inefficient. Due to pressure drops that occur between the traveling and standing valves within the subsurface pump in the suction phase, part of the gas separates from the oil and creates a gas chamber between the traveling valve and the oil flowing across the standing valve. Since both valves require a pressure differential for the ball to separate from the seat, it is necessary to compress the gas during the discharge phase until the gas pressure inside the pump cylinder exceeds the pressure of the fluid column downstream the traveling valve. In most cases, the mobile component can plummet the oil causing a strong fluid pound effect, harming the pump and decreasing its lifespan. Attempts have been made to overcome this problem, including affixing an annular valve to the discharge end of the cylinder to support the counter pressure generated by the weight of the oil column, significantly reducing the pressure differential required to open the traveling valve by the gas trapped between the traveling valve and the liquid phase of the crude oil and increasing, to some extent, the volumetric efficiency of the pump.
Attempts have also been made to address the low volumetric efficiency when pumping fluids with high gas to oil ratio. For example, valves having a single plug and seat have been developed where the plug (directly connected to the sucker rod string through a rod) is forced to move with a reciprocating motion induced from the surface by a beam or other pumping unit, while the plunger moves freely between the plug and a stop. In this case, the plunger can have a seat attached to it, such that whenever the plug contacts the seat, a seal is formed, and when they separate the fluid is able to flow. In such systems, the plug can be separated from the seat due to: (i) the weight of the sucker rod string which acts directly on the plug, (ii) the pressure differential between the suction side and the discharge side of the traveling valve, and (iii) the friction between the plunger and the pump barrel acting on the moveable component. Such valves can open much faster and are more efficient than ball and seat valves (including subsurface pumps with annular valve); however, annular valves could also be implemented were high gas to oil ratios exist.
Many configurations of pumps having single plug and seat traveling valves exist, including the VR-S™ disclosed in U.S. Pat. Nos. 4,591,316, 4,708,597 and 5,048,604, Canada Patent No. 1,221,875, and the LOCK-NO plunger manufactured by the HARBISSON FISHER Company. U.S. Pat. No. 5,044,395 teaches the implementation of a plug, a seat, and one or several seating rings operable as a check valve that offers minimum pressure drop and the maximum possible flow area in a confined cylindrical space. In such valves, the fluid passes in one direction when the intake end of the plug separates from the discharge end of the first ring, while the intake end of the same ring separates from the discharge end of the subsequent ring or rings, depending if there is more than one ring. If there is more than one ring, the fluid is not allowed to return when the intake end of the plug seals against the discharge end of the first ring, while the intake end of the same ring seals against the discharge end of the second ring, and so on, until the intake end of the last ring seals against the discharge end of the seat. As such, the standing valve using one or more rings between the plug and the seat, and the resulting incremented flow area, enables the valve to handle higher viscosity fluids.
U.S. Pat. Nos. 4,591,315 and 4,740,141 teach composite retention valves located specifically at the intake of the plunger, which opens and closes mechanically for the single plug and seat retention valves, but with much greater flow areas. These composite retention valves have the seat attached to the suction end of the plunger, while the rings and the reciprocally actuated plug (by means of a rod that ran across the plunger) are altogether outside said plunger.
Such single plug and composite retention valves require that the traveling valves, rather than the plunger, plunge into to the liquid phase of the fluid within the pump chamber. However, if the intake end of the plunger contacted the fluid before the traveling valve, then the drag force acting on the plunger could aid in an earlier opening of the valve and at the same time extend its useful life, since the fluid pound would be on the plunger and not on the sealing elements of the valve.
Conventional pumps have been somewhat successful to meet the pumping requirements of fluids with high gas to oil ratio, produced in vertical or slightly deviated wells; however, known pump designs can become somewhat inefficient when pumping oil of: (a) very high viscosity, (b) medium or high viscosity with steam due to the injection of steam into the well or adjacent wells to lower the viscosity of heavy and extra heavy crude oil, (c) any viscosity particularly with high gas to oil ratio, or (d) any viscosity in horizontal or highly deviated wells.
There is a need for valve design for increasing the performance of mechanically actuated positive displacement subsurface pumps, the valve being capable of significantly reducing oil seepage and being able to pump a greater amount of fluid. Such a valve may comprise plug and seat sealing elements, where the sealing elements may comprise at least one annular sealing element positioned between the plug and sealing elements. Such a configuration may provide for a considerable increase in the valve flow area.